Heat-mirrors that reflect radiation in the infrared spectrum while transmitting radiation in the visible spectrum have important applications for example as windows in buildings or vehicles.
For transparent heat-mirrors, visual light transmittance must be high, and hence the reflectivity and absorptivity must be low.
In the United States of America for example, automotive windshields must have a transmittance of visible light of at least 70%.
In the infrared, however, the heat-mirror must have high reflectivity and so transmittance and absorptivity in the infra-red must be low.
Heat-mirrors comprising a stack of alternating dielectric and metal layers are known in the art.
To obtain a heat-mirror characterised by a low heat transmittance, generally at least three metal layers are necessary. However, the number and the thickness of the metal layers have a negative influence on the visual light transmittance and on the cost and complexity of the manufacturing process.
It is well known to use silver as metal layer. However, a silver layer has a low stability, low durability and poor moisture and weather resistance.